It is increasingly recognised that integrated optical circuits have a number of advantages over electrical circuits. However, it has been difficult to produce integrated optical circuits that are comparably small, primarily due to the difficulty in producing waveguides that can include tight bends without large signal losses. It has also been difficult to produce integrated optical circuits including signal processing devices which can be easily coupled to current optical fibres, owing to a difference in the refractive index of the material used for optical fibres and those materials typically used for integrated optical devices, whilst still maintaining compact sizes.
Optical signals may be resonantly confined and manipulated using structures whose periodicity is of the same scale as an optical wavelength. Much recent interest has centred upon the field of photonic crystal (PC) structures.
Waveguiding photonic crystal structures are typically based on some perturbation in dielectric constant in the core of a planar waveguide structure. This has most commonly been performed by the spatially periodic etching of air rods through a cladding layer into the core layer of the waveguide. As light propagates through the core, it interacts with the dielectric constant modulation and, in some structures, certain electromagnetic fields are forbidden to propagate in the core, in a manner analogous to electrons in a semiconductor. The forbidden electromagnetic fields form a photonic bandgap. More detail on the nature of the band structure of photonic crystals of this sort can be found in WO 98/53351.
In co-pending patent application (PJF01501US—“Cladding PC”), it was shown that placing a PC in the cladding could provide improved performance for the PC, by reducing the out-of-plane losses and the ability of the PC to couple into near zero group velocity points. Here, the PC in the cladding does not extend into the core layer yet perturbs the evanescent field of an optical signal propagating through that layer.